The accusations were explosive: a head of state had not only begun an illegal war but egged his troops on to a series of horrific atrocities that left thousands dead and an entire continent in ruins. By then, the accused was one of history’s most hated and debated figures, a monarch known for making erratic decisions and doubling down on his sometimes inexplicable actions.

There was just one problem: The accused, Wilhelm II of Germany, couldn’t testify. The accused had been dead for 75 years.

It could have been the trial of the century—if it had been conducted a century before. The trial of Wilhelm II, Germany’s emperor between 1888 and 1918, was a moot one, conducted by historians and legal experts grappling with one of the great mysteries of 20th-century history. Was Wilhelm II guilty of war crimes?

It’s a question that was never answered during Wilhelm’s lifetime. Though the Allies accused him of starting one of history’s bloodiest wars and violating international law, and his troops of committing barbaric acts, he never stood trial. Today, these accusations are remembered as the first stirrings of a modern conception of war crimes. But at the end of World War I, Wilhelm’s responsibility for the bloodshed was a hotly contested—and ultimately unresolved—issue.

The thought of trying him at all was a radical notion. Until World War I, going to war had been seen as the right of any nation or head of state, and war crimes were considered part of the war. A sense of victors’ justice held that atrocities committed by the winning side would go unpunished, while the victors could punish or even execute those on the losing side with impunity.

But World War I changed the face of war—and combat norms—forever. Armed with newly destructive weapons of war like tanks, heavy artillery and gas, both sides sustained huge numbers of casualties while deadlocked in years of trench warfare. Over 6.6 million casualties and 8 million combatants died during the war.

From the beginning, atrocities were part of the Great War. After Germany invaded Belgium in August 1914, German troops began murdering civilians. The massacres, sparked in part by the false belief that Belgian villagers were snipers, claimed over 5,000 victims and sparked a fierce debate about which methods of violence were justified during the war.

As the war dragged on, and more and more civilians on both sides died, both the Allied and Central powers accused one another of war crimes. But when the war ended, only one side was taken to task for its conduct.

Germans believed that the sword had been “forced into [their] hand” by threats from hostile powers. But to the Allies, it appeared that Wilhelm II, Germany’s emperor, had used his power to force the world into war. When the parties gathered at the end of the war to hash out the terms of the Treaty of Versailles, the Allies insisted that Germany not only accept full responsibility for the war but that Wilhelm be tried as a war criminal. As part of the treaty, the signatories agreed to assemble a special tribunal to try Wilhelm for “a supreme offense against international morality and the sanctity of treaties.” The Allies formally requested his extradition.

It was an unprecedented move: Never before had a head of state been tried for starting a war, and though Germany was the loser it still had alliances and ties throughout Europe. At the time, Wilhelm was in the neutral Netherlands, where he had fled at the end of the war at the invitation of the country’s sympathetic Queen Wilhelmina. There, he lived in splendid exile at a former castle filled with his valuable possessions.

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Despite pressure to extradite the Kaiser for trial, the Dutch refused to comply, claiming it would compromise their neutrality to take sides. The Kaiser’s guilt had become an article of faith to the Allies; in both France and Great Britain, it had been an important election issue, with Britain’s new prime minister, David Lloyd George, promising to both try and hang the Kaiser. But quibbling and infighting among the Allies, along with the Dutch refusal to extradite Wilhelm, meant he was never extradited or tried.

The treaty did result in other trials, though. In 1921, Germany’s highest court began a series of war crimes trials in Leipzig. The Leipzig Trials only affected a tiny fraction of the people initially accused of war crimes that ranged from looting to abusing prisoners. But no one could agree whether the trials were, in fact, legal, and the German court used German laws, not international ones, in the trials. No one was happy with the outcome. For the Allies, the Leipzig trials were an empty charade. For Germans, they were victors’ justice, a blatant overreach of international boundaries.

The trials ended with a whimper: Though over 901 cases of war crimes were identified, only 17 were tried. By the late 1920s, the rest had been dropped and few who had been convicted served any time whatsoever. Though the Leipzig trials and the debate over Wilhelm’s war guilt helped set the stage for both international law and the prosecution of war crimes, they failed to achieve the goal of actually punishing anyone who committed or enabled atrocities during the war.

Wilhelm was never tried and died in exile in 1941. Historians are still split on his role in causing World War I. So how did the deposed Kaiser fare in his posthumous “trial”? The verdict was mixed. Though the historians found him guilty of causing German troops to invade neutral Belgium, they acquitted him on all other counts.